Method for the preparation of uranium oxide sols

ABSTRACT

A process for preparing sols of urania composed of particles in the 60 to 1900 angstrom size range which comprises electrodialysizing a solution of a tetravalent salt of uranium in a multi-compartment cell wherein the compartments are separated by an ion exchange resin membrane.

United States Patent 1151 3,691,087 Hurley et al. 1*S8PL 12, 1972 [54] METHOD FOR THE PREPARATION OF 252/313 R URANIUM OXIDE SOLS [51 Int. Cl .......C09k 3/00 [72] Inventors: Forrest R. Hurley, Ellicott City, [58] Field of Search 252/309 2 291 Md.; Melvin Tecotzky, Palo Alto,

Califi; Milton C. Vanlk, Severna Park, Md [56] References Cited 73 Assignee: w. R. Grace & (36., New York, A UNITED STATES PATENTS NY. 3,330,772 12/1964 Fitch et al ..-...,.....252l30l.l 3,091,592 /1963 Fitch et a1 ..252/30l.l The the 3,280,011 /1966 OConnor et al...252/30l.1 x P i= May 1980 3,325,419 6/1967 Hurley et al. ..252/301.1 has been dlsclalmed.

22 Filed; 20, 19 3 Primary Examiner-Carl D. Quarforth Assistant Examiner-F. M. Gittes [21] PP N04 706,773 Attorney-Joseph P. Nigon and Kenneth E. Prince Related US. Application Data 57] ABSTRACT [63] gg g g 3 513 3 5 June A process for preparing sols of urania composed of 3 2 a 32 particles in the 60 to 1900 angstrom size range which tmuauomlmpa" of 9' comprises electrodialysizing a solution of a tetravalent 1963 abandoned whlch a commuanonm' salt of uranium in a multi-compartment cell wherein P of 11159 1960 aban the compartments are separated by an ion exchange donedresin membrane.

[52] US. Cl. ..252/l.1 S, 23/355, 20411.5, 4 Claims, N0 Drawings METHOD FOR THE PREPARATION OF URANIUM OXIDE SOLS This application is a continuation-in-part of application Ser. No. 558,616, now U. S. Patent 3,375,203, filed June 20, 1966, which is a continuation-in-part of application Ser. No. 290,170, filed June 24, 1963, now abandoned, which is a continuation-in-part of application Ser. No. 1159, filed Jan. 8, .1960, now abandoned.

This invention relates to stable urania sols and to methods of preparing these sols. In one specific aspect it relates to a urania sol which can be encapsulated and used in an aqueous homogeneous reactor or can be used in certain ceramic applications. By urania sols, we mean sols of uranium dioxide.

The compound U has a fluorite crystal structure. When the U0 adsorbs oxygen the fluorite structure is stable until the composition reaches approximately U0 (the fluorite structure is reportedly also stable down to the composition U0 Uranium dioxide when heated in air will transform to U 0 Above UO the fluorite structure breaks down. The volume expansion of the U0 transforming to U 0 may cause certain difficulty especially when the uranium dioxide is being used as the fuel source in a reactor.

The utility of urania sols in nuclear fuel development has been well established. These sols have use in the preparation of ceramic fuel elements as well as in the preparation of fuels for nuclear homogeneous reactors. These sols are also frequently used as one of the components of mixed sols with other compositions such as the rare earths, zirconia, etc. Urania sols may be mixed with ceramic powders, especially those of zirconia and yttria to prepare ceramics with improved properties of strength and grain growth control.

We have found it is now possible to prepare urania (uranium dioxide) sols with a suitable urania content, free from undesirable neutron capturing components, suitable for the uses outlined above. The particles or miscelles of the urania sols prepared in accordance with the present invention are of such small size that there is only slight tendency to settle and attrition is not a problem.

For use in aqueous homogeneous reactors, we prefer to coat or clad these sols with silica to improve their hydrothermal stability. The preferred process of coating or cladding comprises the addition of a layer of reactive silica to the sol particles followed by deactivation by treating under non-evaporative conditions, either in an autoclave or under total reflux, and stabilizing by the addition of an alkali metal hydroxide. The urania sols clad with silica according to this process are particularly desirable in that the silica and alkali metal have low neutron capture cross-sections and thus are not a factor in neutron economy. However, in order to obtain the desired characteristics, the urania sol cladding step should be carried out in a carefully controlled manner and under carefully controlled conditions.

Sols of the present invention are composed of urania particles that, from electron microscope studies generally take the form of cubes, although some are roughly spherical. These particles frequently evidence a substructure composed of small cubes packed in a cubic array.

The particle size range of these sols is about to 1900 Angstroms. However, the particles are of generally uniform size and very few fall outside the range of 200 to 600 Angstroms. The particles are in the appropriate size to exhibit colloidal properties.

In the present process, we start with an aqueous solution of a uranium (IV) salt which is sufficiently soluble so that 5 to 10 percent equivalent U0 sols can be prepared. Uranium (lV) nitrate and uranium (IV) chloride have been considered for this purpose. Uranium (IV) chloride gives satisfactory results. Uranium (IV) nitrate is not as satisfactory because of a tendency of the nitrate ion to oxidize the uranium (lV) ions to uranyl ions. The uranium (IV) chloride is better suited for this purpose because uranium (IV) in the presence of chloride ion is stable in solution. Suitable urania sols can also be prepared from uranium tetraformate and uranium tetrabromide.

Apart from the problem of oxidation, there is a secondary problem that must be considered in selecting the salt to be used in preparing the sols. The sulfate, for instance, forms a soluble complex so that the urania passes through the anion exchange resin membrane on electrodialysis. For this reason, the sulfate is not a satisfactory starting material. The carbonates and fluorides also exhibit this property and thus cannot be used to prepare urania sols directly. Preferred techniques suitable for anion removal are electrodialysis using anion permeable membranes, or dialysis using anion permeable membranes.

In the preferred process, we start with an aqueous solution of the uranium salt containing the equivalent of up to 10% U0 This solution is passed into the cathode compartment of an electrodialysis cell wherein the compartments are separated by an anion exchange resin membrane. The electrode compartments are equipped with stirrers and the solution circulated through the cathode compartment and then through a densification vessel maintained at temperature of about to C. The solution is cooled before being returned to the cathode compartment of the cell since the anion exchange resins are damaged by high temperatures, the temperature in the cell is maintained at about room temperature, from 20 to 30 C. The evaporation losses can be made up by adding water to the densification vessel to take care of these losses.

Circulation through the cell is maintained for a period of about nine hours. During electrodialysis, the amperage drops drastically from about 10 to a value of about 0.2 to 0.3. The pH of the solution rises from about 0.5 to 4.

The anode and cathode compartments of the cell are separated by an anion exchange resin membrane. These anion exchange resins and anion exchange membranes are standard articles of commerce. They can be purchased from several vendors. Suitable anion permeable membranes and a method of making the same are described in the following U. S. Pats: Walter J uda et al No. 2,636,851; J. T. Clark No. 2,732,350; J. T. Clark No. 2,730,768; and J. T. Clark No. 2,800,445. The membrane made from the anion exchange resin designated Amberlite IR 45 gives satisfactory results.

The current for the electrodialysis is passed through the solution using electrodes of a noble metal to preclude the possibility of contamination of the product with the metal. Platinum electrodes are most frequently used. The electrodes are positioned a suitable distance, generally about Va inch, from the membrane during electrodialysis.

The urania sols of our invention are characterized by relatively dense particles having colloidal dimensions and exhibiting no tendency to agglomerate at ambient temperatures. Prolonged heating of these particles may, under certain conditions, cause them to accrete with the result that they are not sufficiently stable to hydrothermal treatment. This problem can be solved by cladding the sol micelles with silica. The silica cladding, as stated previously, renders the particles stable enough to be used as fuels in aqueous homogeneous reactors. As verified by electron microscopy, a coating of about 30 to about 120 Angstroms thick can be deposited on the particles using the techniques set out in Example V.

The halogen content of these sols can be reduced to a low value by treatment of the sols with a mixed bed ion exchange resin under appropriate conditions. A urania sol prepared as described can be concentrated by evaporation to a solids content of at least 17 percent. It is preferred to add fresh sol continuously during the evaporation to avoid deposition of the solid material on the sides of the vessel. Concentration to still higher solids content can be effected by centrifuging the sols, removing the solids and redispersing them in a smaller volume of water. This technique is also useful when it is desired to change the solution medium from regular water to heavy water. Sols clad with silica can be concentrated to even higher solids content than the unclad sols. The finished sol may be diluted to any lower solids content by addition of deionized water or water of low ionic content.

Sols, when subject to aging or certain types of heat treatment, undergo a densification in which the open gel structure is altered and the density of the individual particles is increased. Densification of the urania particles is obtained by controlling conditions during the formation of the sol or, to some extent, by subsequent treatment. The maximum concentration of urania which may be obtained in the sol is primarily dependent on its pH, particle shape and size distribution, particle density and, when the so] is clad, the urania to silica ratio.

Since the sols of this type tend to coagulate on the addition of electrolytes, care must be taken to keep the electrolyte content at a minimum. A convenient method of measuring concentration of the undesired ionic material is conductivity. For the sols of the present invention, conductivity will usually range between 10 and mhos./cm. The stability of any given so] is improved by reduction in the electrolyte content. Therefore, conductivities in the lower part of the range are preferred. The sols have a pH in the range of 1.0 and 5.0, preferably 2.5 to 4.0.

Conductivity is measured at 25 C. and one kilocycle using a standard conductivity bridge with a cell inserted in one arm. The cell constant is determined using KCL solutions of 0.01 to 0.02 normality (the conductivity of which is ascertained from the conductivity tables) and using the equation:

K: LKCL R where K the cell constant in cm R bridge resistance in ohms L conductance in mhos./cm. of the standard KCL solution. The conductance L of the sol in question can be determined by measuring its resistance in the same cell and using the equation:

where K cell constant R resistance in ohms.

The viscosity of the sols is determined with an Ostwald viscometer. The viscometer is kept in a constant temperature bath at 25 C. and a 10 ml. sample is used in the viscosity studies. The relative viscosity of a sol can be determined by using water as the standard and assuming the viscosity of water is l. The viscosity is then calculated using the formula:

where n the relative viscosity of the sol n l (viscosity of water) d the density of the sol at 25 C.

t, the time of flow in seconds of the sol in the viscometer d 0.997 (the density of water at 25 C.)

: the time of flow in seconds of water in the viscometer After the relative viscosity has been determined, the absolute viscosity in poises can be obtained by multiplying the relative viscosity by the absolute viscosity of water at 25 C. This value is 0.00895 poises.

In referring to our dispersions of urania in water, we mean to include heavy water as well as natural water. In referring to our dispersions of urania, we intend to include U-233 and U-235 as well as natural uranium which is principally U-238. Our definition of the term urania covers only uranium dioxide UO The present invention will be further explained by the following illustrative but non-limiting examples.

EXAMPLEI A series of runs were completed in which a urania so] was prepared from a nominal 1 percent uranium salt solution.

In a typical member of this series, 3505 ml. of UCL, solution containing 7.73 grams of uranium was charged into a heated vessel for use in preparing a urania sol designated in this specification as a densification vessel. The solution was circulated at a rate of approximately 150 cc./min. to the cathode compartment of a cell separated from the anion compartment by an ion exchange resin membrane of Nepton Ar-lllA. The electrode compartments each had a capacity of approximately 350 ml. and each was equipped with a stirrer. Platinum electrodes were positioned on each side of the membrane a distance of inch from the membrane. The temperature in the densification vessel was maintained at about C. Uranium chloride solution was withdrawn from the densification vessel at the rate of about cc./min. through a cooled heat exchanger and pumped into the above described cell. The temperature of the incoming solution was controlled to maintain a cell temperature of about 18 to 27 C. The solution leaving the cell was passed through a heat exchanger where it was heated to about 80 C. and then returned to the densification vessel. Evaporation losses were minimized by equipping the cell with a condenser and periodically adding deionized water to take care of unavoidable losses. Circulation was continued for a period of nine hours. During the electrodialysis the amperage dropped from about 10.3 to a value of about 0.2 and the pH rose from a value of about 0.5 to about 3.9.

The sol had a density of 1.003 grams/mL, a specific conductance of 1.36 X mhos/cm. and contained 7.37 grams of uranium per liter. A sample of the product was submitted for x-ray analysis. The x-ray diffraction pattern of the particles making up this sol (after drying) showed the following two lines:

d l/Io 3.13 100 2.71 29-39 The particle" size ranged from about 60 to about 700 Angstrom units, but most of the particles were in the size range from 350 to 535 Angstroms. Only the very smallest particles departed from the generally spheroidal form. The data for the other four runs in this series is summarized in Table I and II below.

Concentration is given in grams per liter and density in grams per milliliter.

Specific conductance in mhos per centimeter and absolute viscosity in poises.

These materials after drying showed the two principal lines of the fluorite structure.

TABLE II Specific Amperage Cell Pot. Conduct- Run At on com- Temp. Time Temp. ance of Start pletion Sol 1 10.2 0.6 27 6% 80 3.4)(10' 2 2.7 0.5 25.2 9 80 3.3 10" 3 10 0.2 18-26 9 60 4.7 l0 4 0.25 22.5-27 11 I00 3.89xt0- Temperature is given in C., time in hours and conductance in mhos per centimeter.

It is apparent from these data that a satisfactory urania sol can be prepared from a nominal 1 percent uranium solution.

EXAMPLE II Two runs were completed in which a urania sol was prepared from a nominal 5 percent uranium as a nitrate solution. In a typical member of thisseries, the uranyl nitrate (uranium in the plus VI oxidation state) was reduced to uranium (IV) and converted to the chloride.

A charge of 450 grams of uranyl nitrate hydrate was dissolved in 4 liters of deionized water. This solution was charged into a heated densification vessel maintained at C. for the preparation of a urania sol. The solution was circulated at a rate of approximately cc./min. to the cathode compartment of a cell separated from the anode compartment by an ion exchange resin membrance of Nepton AR-lllA. The electrode compartments each had a capacity of approximately 350 ml. and each was equipped with a stirrer. Platinum electrodes were positioned on each side of the membrance a distance of )4; inch from the membrane. The temperature in the densification vessel was maintained at about 100 C. Hydrochloric acid was added to the solution during electrodialysis to replace the nitrate ion removed by chloride ion. As electrodialysis continued, the uranyl ion was reduced to uranium (IV) and the chloride ion was removed. Circulation was continued for a period of nine hours. During the electrodialysis, the amperage dropped from 10 to a value of about 0.7 and the pH rose from about 0.4 to 3.7. The sol had a density of 1.033 grams/ml. The size of the urania particles in the sol ranged from 200 to 1700 Angstroms. These particles which were approximate spheres, showed some cubic characteristics. Other data collected in this and the other runs in this series are presented in Tables III and IV.

TABLE III Sol Final specific Viscosity Run Concent Density pH conduct- Relative Absolute ration ance to H O U CI Concentration is given in grams per liter and densities in grams per milliliter.

Specific conductance in mhos per centimeter and the absolute viscosity in poises.

TABLE IV Specific Amperage Cell Time Pot U Conduct- Run At on comtemp. Temp. Conc. ance of start pletion Sol Temperatures are given in degrees Centigrade, time in hours, the uranium concentration in grams per liter and the specific conductance in mhos per centimeter.

EXAMPLE III 80 C. and the cell temperature at 26 to 30 C. during the run. The finished sol contained 62.4 grams uranium per liter, its pH was 2.93, its specific conductance 1.193 X mhos/cm., and its absolute viscosity 0.01 poises.

EXAMPLE lV Briefly, the method of coating the urania particles comprises reacting the urania sol with a silica sol. In a typical run a silica sol containing 2 percent SiO was prepared by passing a sodium silicate solution through a column containing Dowex 50 cation exchange resin in the hydrogen form.

The so] to be clad was heated to 40 C. with stirring, then the silica sol was added rapidly with good mixing. The pH of the mixed sol dropped on the addition of the silica sol. The solution was then adjusted to a pH of 9 by the dropwise addition of 1 normal sodium hydroxide. The weight ratio of U0 to Si0 in the cladding operation was 2.2 to l. The mixed sol was refluxed overnight with stirring and after cooling was passed through a mixed bed ion exchange resin and deionized. The pH of the so] was then brought up to 8 by the dropwise addition of 1 normal sodium hydroxide. The clad sols were then placed in a glass pressure vessel and autoclaved at 300 C. for various periods of time in an atmosphere of nitrogen to determine their hydrothermal stability. An electron micrograph of the clad particles showed each particle was coated with a less dense material and that the thickness of the coating was about 30 to 120 Angstroms. Some of the physical and chemical data on these sols is presented in Table V.

TABLE V Run 1 2 3 4 65 hrs. 65 hrs. 200 hrs. 300 hrs.

Obviously, many modifications and variations of the invention as hereinabove set forth may be made without departing from the essence and scope thereof and only such limitations should be applied as are indicated in the appended claims.

What is claimed is:

1. A process for preparing a urania sol by electrodialyzing a solution of a tetravalent salt of uranium in a multi-compartment electrodialysis cell, wherein said compartments are separated by anion exchange membranes which comprises:

a. Preparing a solution of said tetravalent uranium salt selected from the group consisting of the chloride, the bromide, and the formate to have a uranium content calculated as U0 of l to 10 percent;

b. Circulating said solution from a densification vessel through the cathode compartment of said electrodialysis cell while maintaining the temperature in said cell at about 1 5 to 30 C.;

c. Continuing said circulation until a urania sol is formed; and

d. Densifying said sol by heating to a temperature of about 60 to C. and recovering the product sol.

2. A process for preparing a urania so] by electrodialyzing a solution of a tetravalent salt of uranium in a multi-compartment electrodialysis cell, wherein said compartments are separated by anion exchange membranes which comprises:

a. Preparing a solution of said tetravalent uranium salt selected from the group consisting of the chloride, the bromide, and the formate to have a uranium content calculated as U0 of l to 10 percent;

by Circulating said solution from a densification vessel through the cathode compartment of said electrodialysis cell while maintaining the temperature in said cell at about 15 to 30 C.;

0. Continuing said circulation until a urania sol is formed and recovering said sol product.

3. The process according to claim 2 wherein the tetravalent uranium salt is uranous chloride and the densification vessel is maintained at an elevated temperature.

4. The process according to claim 2 wherein the electrodialysis is carried out using platinum electrodes and the solutions in the compartments is stirred. 

2. A process for preparing a urania sol by electrodialyzing a solution of a tetravalent salt of uranium in a multi-compartment electrodialysis cell, wherein said compartments are separated by anion exchange membranes which comprises: a. Preparing a solution of said tetravalent uranium salt selected from the group consisting of the chloride, the bromide, and the formate to have a uranium content calculated as UO2 of 1 to 10 percent; b. Circulating said solution from a densification vessel through the cathode compartment of said electrodialysis cell while maintaining the temperature in said cell at about 15* to 30* C.; c. Continuing said circulation until a urania sol is formed and recovering said sol product.
 3. The process according to claim 2 wherein the tetravalent uranium salt is uranous chloride and the densification vessel is maintained at an elevated temperature.
 4. The process according to claim 2 wherein the electrodialysis is carried out using platinum electrodes and the solutions in the compartments is stirred. 